1. Field of the Invention
The present invention relates to an antenna structure and to a radio-controlled timepiece that uses the antenna structure, and more particularly it relates to an antenna structure especially for a resonance antenna, which is configured so that even in the case in which the antenna structure is disposed in the vicinity of a metal object, the receiving performance of the antenna structure does not decrease, and to a radio-controlled timepiece that uses the antenna structure as mentioned above.
2. Related Art
In recent years many wristwatch products using radio signals have appeared.
Specifically, known products include a wristwatch with a radio in which a radio function is added within the wristwatch so as to obtain prescribed information by receiving a broadcasted radio signal, a radio-controlled timepiece that receives a standard radio signal onto which is superimposed a time code so as to automatically adjust the time of the wristwatch to the standard time during use, and a remotely controlled wristwatch.
In a wristwatch, however, in order to use a radio signal an antenna and receiving circuit are necessary, and it is not only necessary to use components and a design that are completely different from watches in the past, but also to consider the issue of not hindering receiving performance.
Specifically, there is the problem of how to improve the antenna receiving performance and also the restriction in designing with regard to size and design because of the placement of the antenna inside or on part of the outer case of the wristwatch.
In particular the antenna, which greatly influences the performance of receiving a radio signal, has a size that is considerable larger than the other components of wristwatches of the past and further, since a placement of the antenna is restricted with respect to the receiving performance thereof, various methods were used, such as internal mounting, external mounting, extendable/retractable mounting, or cord-type mounting.
The internal mounting method is generally used with a bar type antenna formed by a magnetic core and a coil, and when mounting within a wristwatch, it is necessary to take care with regard to the case material and structure, and with regard to design, in order that the receiving performance is not decreased.
In the case of external mounting, in a method such as the extendable/retractable method used in radio-cassette recorder combinations or a cord-type method in which a cord is also used for earphones or the like, it is necessary to consider the overall design, storability, and endurance and the like of the watch.
In this situation, in order to achieve not only compactness and thinness, but also fashionable appearance in a wristwatch, it is of course necessary to give sufficient consideration not only to not causing a drop in receiving performance of the antenna, but also to ease of portability and designability, thereby leading to a demand to make the antenna smaller.
In a radio-controlled timepiece it is the antenna characteristics and the receiving circuit characteristics that determine the receiving performance, the lower limit of the signal input to a receiving circuit or a receiving IC being a signal amplitude of approximately 1 μV at present, so that in order to achieve practically useful receiving performance, it was necessary to obtain an output having a signal amplitude of approximately 1 μV with an antenna in an electrical field strength (strength of the radio waves) of 40 to 50 dBμV/m.
For this reason, in the case of a size restriction, a resonant-type receiving antenna, which enables the achievement of a large signal output, is generally used, and further, regarding the type of the receiving antenna, since the wave length of the radio wave is long, a bar antenna in which conductive wire is wound on a magnetic core is typically used.
With this type of receiving antenna, because the output of the receiving antenna is approximately proportional to the size of the receiving antenna, it is not possible to make a size of the antenna too small in order to obtain practically usable receiving performance.
Accordingly, there are problems in selecting a material or in positioning of a member used in a place or in the vicinity of the antenna so that the receiving characteristic should not be reduced in the case of a compact wristwatch.
In particular, because there is an extreme decrease in the output of the antenna when it is housed in a metal outer case, consideration is necessary so that receiving performance is not hindered.
For this reason, in order to use a radio signal in a wristwatch, it is necessary not only to use the component structures and a design that are completely different from those of watches in the past, but also to consider the issue of not hindering receiving performance.
In the case of a radio-controlled timepiece of the past, the mounting of the antenna was generally made by external mounting method and by internal mounting method, and in the case in which the outer case comprising a bottom cover part and a side part was made of metal, mounting of the receiving antenna was generally made outside.
Because a non-metal such as plastic or the like was used in order that the antenna case does not cause a decrease in receiving performance, there was a large protrusion, so that not only were compactness, thinness, and portability lost, but also there was a prominent loss in the degree of freedom of design.
Also, in the case of an internal receiving antenna, although ceramic or plastic is used as a material for the outer case (bottom cover part and side parts) of the wrist watch in order not to reduce the receiving performance, because these materials have little strength, the thickness thereof increases, thereby causing a loss of housing capacity and portability, and also greatly restricting design, resulting in a wrist watch that is lack of high quality feeling with massive feeling in its appearance.
For this reason, in the past, for example as can be seen in Japanese Unexamined Utility Model Publication No. 2-126408, a metal antenna has been disposed within a leather band of the watch.
Also, as disclosed in Japanese Unexamined Utility Model Publication No. 5-81787 by the applicant of this patent application, there an instance in which an antenna in which a coil is wound around a core is disposed between the dial plate and the windshield, which distances it from the metal outer case itself that would interfere with the radio waves and also provides a unique design, and in international patent disclosure WO95/27928, there is the disclosure of the mounting of an antenna on the side part of a watch case of a wristwatch.
Additionally, in European patent disclosure 0382130, there is also a disclosure of the disposition of an antenna for example on the top surface of a case in a ring shape.
However, in a configuration in which the antenna is disposed in the band, because the antenna exists inside the band, it is necessary to make electrical connection with the electronic apparatus, and it is not possible to impart sufficient flexibility to the connection part between the two.
Additionally, it is not possible to use a band of metal, which would interfere with radio waves, and it is necessary to use a band of rubber or the like, this presenting a restriction in terms of materials and design.
In a configuration in which the antenna is mounted on the upper surface or side surface of a wrist watch because the antenna is at a distance from the metal part of the wrist watch itself, there is an increase in the thickness or size of the overall watch, thereby causing a problem of a design restriction.
Additionally, in the instance in European patent 0382130, in which the antenna is disposed in a ring shape on the upper surface of the case, because reception is not possible if metal exists within the ring, there is the problem of the practical necessity to provide antenna that is separate from the watch.
Additionally, although in Japanese Unexamined Patent Publication No. 11-64547 there is a disclosure of a wristwatch in which a coil is disposed in a channel-shaped depression provided around the periphery of a circuit board and in which a core is disposed in a curve along the circumferential direction of the circuit board, in addition thereto the manufacturing process thereof is made complex, and further the assembly process in the manufacturing process also becomes complicated leading it troublesome.
In the Japanese Unexamined Patent Publication No. 2001-33571 or Japanese Unexamined Patent Publication No. 2001-305244 and the like, there is disclosure of a wristwatch in which the windshield and bottom cover part are made of a non-metallic material such as glass or ceramic or the like, and a metal material as in the past is used therebetween so that sufficient radio waves reach the antenna.
Specifically, in the above-noted examples of the past, the output of the receiving antenna was based on a fact that it is extremely reduced when the antenna is externally mounted on a metal outer case, and the object is to make the material of the bottom cover with non-metallic so as to reduce the drop in output and use sides of a metal that has a high massive feeling in its appearance.
In the above-noted prior example, however, because glass or ceramic is used, there is the problem that the thickness of the watch increases. Also, because either a large sized high-sensitivity antenna structure was used or usage thereof was limited to an area in which the radio signal field strength was high, the convenience of the radio-controlled timepiece is suffered.
Furthermore, in a wristwatch having this configuration, although it was possible to achieve a radio signal that reached the antenna, and the bottom cover part was thinly plated with a metallic plating so as to give the user the impression that metal was actually being used, in terms of outer appearance, there was no feeling of weightiness or textural quality, so that the high-quality image was lost.
For this reason, compactness, thinness, portability, freedom of design, massive feeling in appearance (feeling of high quality) are important factors and, in spite of a demand for a type with a metal outer case and a built-in antenna, in the past there were no radio-controlled timepiece with a fully metal outer case and a feeling of high quality.
Also, in the past, as shown in FIG. 3, in the case in which an antenna structure 102 for the purpose of receiving an external radio signal is disposed on the inside of a metal outer case 103 having electrical conductivity, for example, inside the side or bottom cover (hereinafter collectively referred to as the metal outer case in the present invention) used as the outer case of the watch made, for example, of stainless steel, titanium, or a titanium alloy or the like, considering that the magnetic flux 104 of the external radio signal are absorbed by the metal outer case 103 so that the external radio signal does not reach the antenna structure 102 and the output of the antenna drops, in order to improve the sensitivity of the antenna structure 102, the antenna structure 102 was made large and, the antenna structure 102 was provided outside the metal outer case 103 or a plastic or ceramic outer case was used instead of the metal outer case 103, and in order to achieve an accompanying improvement in the quality of the outer appearance, a thin metal plating or metallic paint was applied to a non-metallic surface.
The inventors of the subjection invention, however, as a result of further study, discovered that the above-noted understanding of the problem in the past was in error, and that even if the antenna structure 102 is disposed within a metal outer case 103 of metal that has electrical conductivity, the external radio signal substantially reaches the antenna structure 102, the problem being, as shown in FIG. 3, that the magnetic flux 105 generated from the antenna core part 106 of the antenna structure 102 at the time of resonance interacts with the metal outer case 103 (as eddy current loss), so that there is a resulting loss of magnetic energy, causing the Q value of the resonant antenna to decrease, so that there is a decrease in the voltage output of the antenna structure 102, thereby greatly reducing the receiving performance.